The buccal ganglion of Aplysia californica contains a population of multiaction interneurons (S1's) which: a) mediates four types of postsynaptic potentials; b) exhibits great redundancy of connections; and, according to preliminary data, c) utilizes a transmitter which is not ACh, 5H%, Hm, DA, or GABA. Many of the S1 follower cells also receive inputs from identified neurons known to be cholinergic and serotonergic interneurons. The objectives of the proposed research are to: 1) Ascertain the nature of the S1 transmitter by: a) comparing the ionic conductance mechanisms underlying the four types of synaptic responses with those evolved by the application of potential candidates; b) comparing the antagonist sensitivities of the synaptic and pharmacologically-evoked responses; c) bioassay of CNS tissue fractions by application to each of the four types of S1 follower cells; and d) chemical fractionation of the presynaptic neurons, and scanning for unique constituents using gas chromatography-mass spectrometry techniques. 2) Demonstrate capabilities for synaptic plasticity in the four types of S1 follower cells by; a) repetitive activity of S1 inputs; b) temporal interactions of several convergent S1 inputs: and c) interactions of inputs from serotonergic and/or cholinergic inputs with S1 inputs. 3) Observe the interactions of S1's and various follower cells in a semi-intact preparation to determine the probable function of the S1's and the significance of the various types of S1-mediated responses.